Method for making double-wall shells by centrifuging

ABSTRACT

A method of fabricating a double-walled shell of fiber reinforced plastic material is by centrifuging. The outer wall of the shell is made by projecting glass fibers and resin in a liquid phase against an inside face of a cylindrical mold rotating about its horizontal axis. A section member is then applied against the inside face of the outer wall by a plurality of turns having a diameter substantially equal to the inside diameter of the outer wall and separated from one another by a distance substantially equal to the thickness of the outer wall. The section member has a channel section with a web and two flanges extending radially outward, defining a sealed helical duct. The space between the turns is filled with resin by centrifuging, forming a helical spacer, and the inner wall is formed by centrifuging.

FIELD OF THE INVENTION

The invention relates to a method of fabricating double-walled shells offiber-reinforced plastics material by centrifuging.

DESCRIPTION OF THE PRIOR ART

In the method of fabricating shells by centrifuging, a wall is made byprojecting glass fibers and resin in the liquid state against the insideface of a cylindrical mold rotating about its axis, which is horizontal.The method is commonly used for making shells of large dimensions,having lengths that may be as much as 12 meters (m) and diameters thatmay be as great as 4.2 m. The mold is constituted by two half-shellsthat are fixed together by bolting in a radial join plane, and it isrotated on a cradle by means of wheels at a speed that may be as high as50 revolutions per minute (rpm) to 60 rpm. The present method issuitable only for obtaining shells or vessels having a single wall bysuperposing a plurality of layers of material. The first layer, referredto as gel-coat, serves mainly to facilitate unmolding, and to give asolid color and decent appearance. Furthermore, by adding additives,this layer can serve to protect the vessel from external aggression.

The second layer is the central core of the wall. It is made up of anassociation of polyester resin and glass fibers. This composite givesthe shell the necessary mechanical strength.

The third and last layer is referred to as a “top-coat”. Its maincharacteristic is to give a smooth surface state to ensure thatsubstances stored in the vessel flow properly. It also serves to protectthe composite material from being attacked by the stored substance.

In order to make the shell, the element that is to constitute the endwall of the vessel is installed at one end of the mold and becomessecured to the shell once it has been made.

The various ingredients are placed inside the rotating mold by using atelescopic arm lying on the axis of the mold. The arm is deployed over12 m so as to enable the ingredients needed for obtaining the compositematerial to be deposited. The resin, which is conveyed by pipes, passesthrough a tub in order to adjust its pressure before being deposited inthe shell.

A grinder installed at the end of the telescopic arm serves to depositfibers of well-defined length. After depositing the substances and theglass fibers, a system of tamping rollers serves to make the compositemore uniform by causing the bubbles of air that are present in themixture to escape. Molding can thus be performed by projectingreinforced resin into the rotating mold. The resin stays in placebecause of centrifugal force. In addition, the tamping rollers debubblethe resin.

Once the shell has been made, all that remains to be done is to unmoldit. For this purpose, the mold is initially opened by moving its twohalf-shells apart, and then the molded shell is extracted by means ofcarriages supporting the ends of the shell and traveling parallel to theaxis of the mold.

The vessels obtained using such large-dimensioned shells are commonlyused for storing grains or chemicals. In this type of storage, the axisof a vessel is vertical.

A need exists to be able to store a variety of substances in undergroundvessels. Under such circumstances, the vessels are laid horizontally,and their walls must be capable of withstanding considerable loads.Clearly it is possible to increase the thickness of the wall, but thatleads to an exorbitant cost for wall material and to a considerableincrease in weight.

WO 93/07073 and EP 0 760 346 disclose methods of making vessels havingtwo concentric walls interconnected by spacers. In those two documents,the inner wall is made around a mandrel, and then the spacers and theouter wall are made. In WO 93/07073, a ribbed foam is applied on theinner wall prior to making the spacers by depositing resin and glassfibers. In EP 0 760 346, a ribbed casing is deposited on the outsideface of the inner wall, thereby forming cavities around the inner wall,followed by a cylindrical casing so that the resin constituting theouter wall does not fill the grooves.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is to propose a method of fabricatingdouble-walled shells in which all of the plastics material is depositedby centrifuging.

The invention thus relates to a method of fabricating double-walledshells of fiber reinforced plastic material by centrifuging, in whichmethod the outer wall is made by projecting glass fibers and resin inthe liquid phase against the inside face of a cylindrical mold rotatingabout its axis, which is horizontal.

The method of the invention is characterized by the fact that after theouter wall of the shell has been made, a metal section memberconstituted by a plurality of turns having a diameter substantiallyequal to the inside diameter of the outer wall and separated from oneanother by a distance substantially equal to the thickness of the outerwall is applied against the inside face of said outer wall, said sectionmember having a channel section with a web and two flanges extendingradially outwards, with their free edges penetrating into the materialconstituting said outer wall so as to define a helical duct that issealed and that extends from one end to the other of said shell, afterwhich the inter-turn space is filled with the resin by the centrifugingmethod in order to form a helical spacer, and the inner wall is made bythe centrifuging method.

Thus, in the method of the invention, the material constituting theshell is a homogeneous composite. The gel-coat layer is initiallydeposited on the inside face of the mold and the top-coat layer isdeposited on the inside face of the composite outer wall.

Most advantageously, the metal section member is applied against theinside face of the outer wall by causing the mold to turn about its axisof rotation while the curved section member is being pressed againstsaid face, said section member being delivered continuously and inappropriate quantity substantially tangentially to said inside face, bya device for fabricating a curved section member from a metal strip, andby displacing said device and said mold axially relative to each otherwhile the section member is being applied as a function of the speed ofrotation of the mold.

Preferably, the delivered curved section member is pressed by means ofpresser wheels carried by the fabrication device.

The invention also provides an installation for implementing the method.

The installation comprises:

a cylindrical mold suitable for turning about its axis, which ishorizontal;

a fabrication device for fabricating a curved channel section member,the device comprising from upstream to downstream:

-   -   a metal-strip dispenser;    -   means for conveying and guiding the metal strip delivered by the        dispenser;    -   means for forming a rectilinear channel section member from the        metal strip;    -   means for curving the channel section member;    -   means for pressing the curved section member against the inside        face of the outer wall; and    -   drive means for driving the transport means, the forming means,        and the curving means; and

means for displacing said fabrication device axially relative to themold, and control means for controlling said drive means and said axialdisplacement means synchronously with the speed of rotation of the mold.

Preferably, the forming means comprise at least three pairs of formingwheels disposed substantially perpendicularly to the axis of rotation ofthe mold.

Advantageously, the curving means comprise a pair of stationary guidewheels for holding the channel section member and preventing the curvingforce being transferred to the forming means, and two curving wheelsdisposed on either side of the web of said section member and offset inthe travel direction of the section member, said curving wheels beingadjustable perpendicular to the travel direction of said section memberin order to deliver a curved section member having a predeterminedradius of curvature, the section member being delivered substantiallytangentially to the inside face of the outer wall.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and characteristics of the invention appear on readingthe following description made by way of example and with reference tothe accompanying drawings, in which:

FIG. 1 is an end view of a mold used for making a shell by centrifuging,together with its support and means for driving the mold in rotation;

FIG. 2 is a section on line II—II of FIG. 1;

FIG. 3 is a section on a radial plane through the mold and the shellmade by the method of the invention;

FIG. 4 is a side view of the installation showing an example of thedevice for making a curved channel section member; and

FIG. 5 is an end view of the device for making a curved channel sectionmember.

FIGS. 1 and 2 show a conventional installation 1 for making asingle-walled shell in accordance with the prior art.

DETAILED DESCRIPTION FO THE INVENTION

The installation 1 essentially comprises a cylindrical mold 2 made up oftwo half-shells 2 a, 2 b that are separable on a midplane containing thehorizontal axis X of the mold and that are suitable for being connectedtogether by bolts 3. The mold 2 presents rings 4 a, 4 b at its peripherythat are used for driving it in rotation. The mold 2 rests on two sidecradles 5 a, 5 b which present pairs of front and rear wheels 6 a and 6b on common axes, with the rings 4 a and 4 b resting thereon. Thecradles 5 a and 5 b also carry wheels 7 a and 7 b on axes that areperpendicular to the axes of the wheels 6 a and 6 b and that run on theflanks of the rings 4 a and 4 b so as to prevent the mold 2 from movingaxially. Two wheels 6 a and 6 b of the cradle 5 b, for example, aredriven by a motor 8, thereby driving the mold 2 in rotation, and alsocausing the six other wheels 6 a, 6 b and the wheels 7 a, 7 b to rotate.

The two cradles 5 a and 5 b can be spaced apart from each other byactuators, e.g. in order to enable unmolding to be performed by openingthe mold 2 after removing the bolts 3, the join plane then beingvertical, and the bottom edges of the two half-shells 2 a and 2 b thenbeing connected to the cradles 5 a and 5 b by cables 9 a and 9 b fixedto hooks 10 on each of said half-shells 2 a, 2 b.

To mold a shell, the two half-shells 2 a and 2 b of the mold are movedtowards each other by moving the cradles 5 a and 5 b towards each other,they are then bolted together and the cables 9 a and 9 b are unhooked.Switching on the motor 8 causes the mold 2 to rotate.

The installation 1 also has a telescopic arm 11 mounted on a structure12 outside the mold 2. The end 13 of the telescopic arm can be movedparallel to the axis X. This end carries a grinder 14 for grinding glassfibers and a tub 15 for receiving a resin in liquid form. In operation,the glass fibers and the resin are sprayed against the inside wall ofthe rotating mold. The liquid resin remains against the wall of the moldbecause of centrifugal force. To build up a given thickness of wall, theend 13 of the telescopic arm is moved back and forth over the entirelength of the mold 2.

FIG. 3 shows the structure of the shell 16 that is obtained by theinvention:

the shell 16 presents from the outside towards the inside:

a) an outer layer 17 of gel-coat deposited by the known centrifugingmethod;

b) an outer wall 18 of composite material, glass fiber reinforcedpolyester resin, said layer being deposited by the known centrifugingmethod described above in the present text;

c) a bonding zone comprising firstly a helical empty space 20 defined bythe inside face 18 a of the outer wall 18 and the inside face of acurved channel section member 21 having a web 22 and two flanges 23 aand 23 b which extend radially outwards and whose edges bear inleaktight manner against the inside face 18 a of the outer wall 18, saidcurved section member 21 comprising a plurality of turns extending fromone end to the other of the shell 16, and also comprising a helicalspacer 24 filling the gap between the turns of the curved section member21, and made of polyester resin possibly filled with glass fibers by thecentrifuging method;

d) an inner wall 25 of composite material, glass fiber reinforcedpolyester resin, deposited by the centrifuging method; and

e) an inner layer 26 of top-coat deposited by the centrifuging method.

The walls 18 and 25 are 7 millimeters (mm) thick, for example, and thethickness of the helical spacer 24 in the axial direction of the mold 2is 7 mm, for example.

The web 22 of the curved section member 21 is 30 mm wide, for exampleand the flanges 23 a and 23 b extend radially over 20 mm. With thesedimensions, the volume of the empty helical space 20 is substantiallyequal to half the volume of the shell 16.

In the method of the invention, after depositing the outer wall 18,removing bubbles from said wall, and allowing it to polymerize at leastin part, the curved channel section member 21 is applied against theinside face 18 a of the outer wall 18 and the helical space between theturns is filled with polyester resin possibly filled with glass fibers,so as to make the helical spacer 24 which is allowed to polymerize atleast in part prior to making the inner wall 25 and the top-coat layer26.

FIGS. 4 and 5 show a device 30 for making and laying the curved channelsection member 21 as described above against the inside face 18 a of theinner wall 18.

The device 30 comprises a structure 31, possibly mounted on rails, whichis disposed outside the mold 2 and which supports a reel 32 of metalstrip and a rigid rod 33 or telescopic arm disposed parallel to the axisX of the mold 2. The rod 33 includes means for transporting and guidingthe metal strip 34 towards the end 35 of the rod 33.

The end 35 of the rod 33 carries means for changing the travel directionof the metal strip 34, so as to enable a device 36 for continuouslyfabricating the curved section member 21 to be fed continuously with themetal strip 34, said device 36 being carried by the end 35 of the rod33.

The device 36 essentially comprises four aligned pairs of wheels on axesparallel to the axis X of the mold, serving to convert the metal strip34 into a rectilinear channel section member of flanges that extendtowards the closest zone of the mold wall and whose path isperpendicular to the axis X. The first pair of wheels 37 enables themetal strip 34 to be guided accurately. The second pair of wheels 38folds the edges of the metal strip to a 30° position, the third pair ofwheels 39 folds them to a 60° position, and the fourth pair of wheels 40folds them to a 90° position.

Downstream from the four pairs of wheels 36, 38, 39, and 40, in thetravel direction of the metal strip 34, there is provided a device 50for adjustably curving the section member, which device comprises afirst pair of wheels 51, that are stationary, for guiding therectilinear channel section member, a first curving wheel 52 bearingagainst the outside face of the web 22 of the section member 21, and asecond curving wheel 53 bearing against the inside face of the web 22 ofthe section member 21 and situated downstream from the first curvingwheel 52.

The two curving wheels 52 and 53 are movable and adjustable in adirection substantially perpendicular to the travel path followed by thesection member 21 so as to enable the radius of curvature of theresulting curved section member 21 to be adjusted as a function of theradius of the mold 2, and so as to enable the curved section member 21leaving the curving device 50 to come substantially tangentially intocontact with the inside face 18 a of the outer wall 18. On leaving thecurving device 50, there are wheels 54 which enable the channel sectionmember 21 to be pressed against the inside face 18 a of the outer wall18 so that the edges of the flanges 23 a and 23 b penetrate far enoughinto the outer wall 18 to ensure that the helical cavity 20 is sealedsufficiently to prevent the liquid resin that will subsequently fill thegap between the turns from penetrating into said helical cavity 20.

In practice, the radius of curvature of the curved section memberleaving the curving device 50 is smaller than the radius of curvature ofthe outer wall 18, and the wheels 54 have a second function ofstraightening the curved section member 21 out somewhat.

The various wheels of the device 36 for preparing the curved sectionmember 21 are rotated by a motor 55 operating synchronously with thespeed of rotation of the mold 2 so as to deliver the curved sectionmember 21 as a function of requirements. The torque delivered by themotor 55 is preferably sufficient to ensure that the delivered sectionmember presses positively against the inside face 18 a of the outer wall18.

While the section member is being put into place, the end 35 of the rod33 is also moved parallel to the axis X relative to the mold 2 inproportion to the speed of rotation of the mold 2 so that the distancebetween turns is constant. This linear displacement can be implementedby means of a motor acting on the length of the rod 33, if it comprisesa telescopic arm, or causing the structure 31 to move axially if it ismounted on rails, or causing the cradles 5 a and 5 b supporting the mold2 to move axially.

Synchronization is advantageously achieved by means of a device forcontrolling the various motors that receives signals from an angleencoder associated with the mold 2.

1. A method of fabricating a double-walled shell of fiber reinforcedplastic material by centrifuging, wherein the double-walled shell has anouter wall having a thickness, an inside face, and an inside diameter,the method comprising: projecting glass fibers and resin in a liquidphase against an inside face of a cylindrical mold rotating about itshorizontal axis, forming the outer wall, applying a curved metal sectionmember against the inside face of the outer wall with a device forfabricating curved section members from a metal strip, wherein the metalstrip is pressed against the inside face of the outer wall and the moldis rotated about its horizontal axis forming a plurality of turns havinga diameter substantially equal to the inside diameter of the outer walland separated from one another by a distance substantially equal to thethickness of the outer wall, said section member defining a channelsection, wherein said channel section has a web and two flangesextending radially outward having free edges wherein said free edgespenetrate said outer wall, defining a sealed helical duct extending fromone end of said shell to the other end of said shell, and wherein thedistance between the turns defines a space, filling in the space betweenthe turns with resin by centrifuging, forming a helical spacer, andforming the inner wall by centrifuging, wherein said section member isapplied continuously and substantially tangentially to said inside faceand is formed as a function of the speed of rotation of the mold, andwherein said device and said mold are displaced axially relative to eachother.
 2. The method of claim 1, wherein the curved section member ispressed by presser wheels carried by the fabrication device.
 3. Aninstallation for implementing a method of fabricating a double-walledshell of fiber reinforced plastic material by centrifuging, comprising:a cylindrical mold for turning about its horizontal axis; a fabricationdevice for fabricating a curved channel section member, the devicecomprising from upstream to downstream: a metal-strip dispenserdelivering a metal strip; means for transporting and guiding the metalstrip delivered by the dispenser; means for forming a rectilinearchannel section member from the metal strip; means for curving thechannel section member; means for pressing the curved section memberagainst an inside face of an outer wall of a fiber reinforced plasticlayer deposited on an inside face of the cylindrical mold; drive meansfor driving the transport means, the forming means, and the curvingmeans; means for displacing said fabrication device axially relative tothe mold, and a synchronization device which synchronizes said drivemeans and said axial displacement means with speed of rotation of themold.
 4. The installation of claim 3, wherein the forming means compriseat least three pairs of forming wheels disposed substantiallyperpendicular to the axis of rotation of the mold.
 5. The installationof claim 4, wherein the curving means comprise: a pair of stationaryguide wheels for holding the channel section member and for preventingthe transfer of the curving force to the forming means, and two curvingwheels disposed on either side of the web of said section member andoffset in the travel direction of the section member, wherein saidcurving wheels are perpendicularly adjustable relative to the traveldirection of said section member and deliver a curved section memberhaving a predetermined radius of curvature, and wherein the sectionmember is delivered substantially tangential to the inside face of theouter wall.